A key parameter to monitor in any laser system is the power of the optical signal produced by the laser system. Conventional optical-fiber-power-measurement systems generally fall into one of two categories: (1) fiber-optic power measurement in which the fiber tip is cleaved or polished and inserted into a measurement head so that all power is read and stopped at the measurement head (also referred to herein as “interruption” power measurement); and (2) fiber optic devices for which the optical fiber is side polished or otherwise incorporated into an “in-line” power monitor that has some non-zero insertion loss.
Interruption power monitors interrupt the optical train, rendering it useless for in situ power measurement, monitoring, or fault detection. Examples of interruption power monitors include the Ophir PD300-IRG Fiber Optic Power Meter Head by Ophir Optronics Solutions Ltd. (www.ophiropt.com/laser-measurement-instruments/new-products/pd300-r), and the Thorlabs S140C with S120-FC Fiber Adapter by Thorlabs Inc. (www.Thorlabs.com/newgrouppage9.cfm?objectgroup_id=3328&pn=S140C#6034).
Conventional in-line power monitors use taps that compromise fiber integrity, risk optical damage at high power, and introduce insertion loss (e.g., conventional in-line power monitors alter the optical fiber in ways that risk damage in high-power applications, degrade optical performance, cause backscatter, and/or reduce power throughput). Examples of in-line power monitors include the EigenLight Series 500 Inline Optical Power Monitor by EigenLight Corporation (www.eigenlight.com/products/portable-optical-power-monitors/series-500), and the FiberLogix Inline Power Monitor (www.fiberlogix.com/Passive/powermonitor.html). In-line power monitors introduce an insertion loss by tapping off some of the power in the core to direct to an optical detector. In-line power monitors operate either by stripping off some optical power exploiting evanescent wave effects or by means similar to optical couplers or splitters. Conventional in-line power monitors are undesirable for efficiency and total power reasons where an in-line device is to remain in place during full operation (e.g., when it is required in change-monitoring systems, power feedback systems, fault detection system, and the like). Some conventional in-line power monitors alter the fiber with a side polish or notch to redirect or probe the power propagating in the cladding. This introduces a weakness in the fiber both mechanically and in laser damage threshold reduction. Alterations of the fiber surface and cladding are undesirable in high-power fiber laser systems. Conventional in-line power monitors that use these tap and other principles of operation lack data for operation higher than 80 watts (W) and have damage thresholds or max operating power specifications in the hundreds of milliwatts (mW) to a few 10's of watts range (e.g., approximately 50 W). Conventional in-line power monitors and their possible faults also introduce safety risks should the system fail and fire or system damage occur. Therefore, conventional in-line power monitors are unsuitable for safely measuring optical power in high-power systems (e.g., about 1 kilowatt or higher).
U.S. Patent Application Publication 2013/0087694 to Daniel J. Creeden et al. (hereinafter, “Creeden et al.”), titled “INTEGRATED PARAMETER MONITORING IN A FIBER LASER/AMPLIFIER,” published Apr. 11, 2013, and is incorporated herein by reference. Creeden et al. describe techniques for monitoring parameters in a high power fiber laser or amplifier system without adding a tap coupler or increasing fiber length. In some embodiments, a cladding stripper is used to draw off a small percentage of light propagating in the cladding to an integrated signal parameter monitor. Parameters at one or more specific wavelengths (e.g., pump signal wavelength, signal/core signal wavelength, etc.) can be monitored. In some such cases, filters can be used to allow for selective passing of signal wavelength to be monitored to a corresponding parameter monitor. The filters can be external or may be integrated into a parameter monitor package that includes cladding stripper with integrated parameter monitor. Other parameters of interest (e.g., phase, wavelength) can also be monitored, in addition to, or as an alternative to power. Numerous configurations and variations will be apparent in light of this disclosure (e.g., system-on-chip).
U.S. Pat. No. 4,586,783 issued May 6, 1986 to Bruce D. Campbell et al. (hereinafter, “Campbell et al.”), titled “SIGNAL COUPLER FOR BUFFERED OPTICAL FIBERS,” is incorporated herein by reference. Campbell et al. describe a signal coupler for buffered optical fibers that comprises a soft, transparent, polymeric rod against which the fiber is pressed by a rigid “key” having regularly spaced protrusions which induce periodic microbending of the fiber. An optical signal passing down the fiber may be coupled into the polymeric rod by the key pressing the fiber into the rod, and the signal extracted from the end of the rod. A similar process may be used to inject an optical signal into the fiber. The coupler may be used either as a termination for a fiber or as part of a non-destructive tap. The induced attenuation and the intensity of the extracted signal may be varied by varying the pressure on the key.
U.S. Pat. No. 4,824,199 issued Apr. 25, 1989 to William D. Uken (hereinafter, “Uken”), titled “OPTICAL FIBER TAP UTILIZING REFLECTOR,” is incorporated herein by reference. Uken describes a tap for withdrawing light from an intermediate portion of an optical fiber core by passing light through a side of the optical fiber comprises an optical coupler in contact with an outside surface of an optical fiber which is bent and disposed in a plane. A light reflector extending transverse to the plane deflects the withdrawn light towards the end surface of a light element disposed completely outside the plane. A similar arrangement may be used to inject light to an intermediate portion of an optical fiber. The tap may be used as a read tap to withdraw light, or as a write tap to inject light in optical fiber networks.
U.S. Pat. No. 6,424,663 issued Jul. 23, 2002 to Bernard Fidric et al. (hereinafter, “Fidric et al.”), titled “POWER MONITOR FOR FIBER GAIN MEDIUM,” is incorporated herein by reference. Fidric et al. describe a fiber optic gain system that has output power monitoring and control using the detected level of side light emitted through the cladding of the gain fiber. The fiber is wound on a spool that is provided with an opening adjacent to the fiber cladding. A photodetector is mounted to the spool at an opposite side of the opening, and detects side light that is transmitted through the opening. An output signal from the photodetector is indicative of the output power or the gain of the system, and may be used for monitoring and/or to adjust the power generated by a pumping source for the system. This allows feedback control of the system that helps to stabilize the output power or gain. A filtering element may also be used to exclude certain undesired wavelengths from the side light being detected.
U.S. Pat. No. 6,744,948 issued Jun. 1, 2004 to Bo Pi et al. (hereinafter, “Pi et al.”), titled “FIBER TAP MONITOR BASED ON EVANESCENT COUPLING,” is incorporated herein by reference. Pi et al. describe fiber tap monitors formed on side-polished fiber coupling ports based on evanescent coupling.
U.S. Pat. No. 7,116,870 issued Oct. 3, 2006 to Craig D. Poole (hereinafter, “Poole”), titled “BROADBAND FIBER OPTIC TAP,” is incorporated herein by reference. Poole describes a broadband optical fiber tap for transferring optical energy out of an optical fiber having an optical fiber with a primary and secondary microbends for the purpose of coupling optical energy into the higher-order modes of the fiber, and a reflecting surface formed in the cladding of the fiber and positioned at an angle so as to reflect, by total internal reflection, higher-order mode energy away from the optical fiber. In the preferred embodiment, the two microbends are spaced apart by a distance approximately equal to one-half of the intermodal beat length for LP01 and LP11 modes of a single-mode fiber.
U.S. Pat. No. 8,452,147 issued May 28, 2013 to Alexey V. Avdokhin et al. (hereinafter, “Avdokhin et al.”), titled “ASSEMBLY FOR MEASURING OPTICAL SIGNAL POWER IN FIBER LASERS,” is incorporated herein by reference. Avdokhin et al. describe a fiber laser system configured with a power measuring assembly surrounding a splice between two fibers. The power measuring assembly is operative to maintain the splice at a substantially constant splice temperature and shield the spliced fibers from external bending stresses so as to provide for power readings of the laser system at the splice independently from the influence of multiple variable external factors.
There is a need for an improved system and method for measuring the power of optical signals propagating through an optical fiber.